Vertical layering
Also said as Vertical remixing, Stems, Layered music
Splitting one piece of music into layers that all play at once in sync, then fading them in and out with the game. The music can change at any moment without a seam.
It responds instantly and cannot go out of time. The catch is that all the layers must work together, so the melody and harmony cannot really change: only the intensity can.
Say it
“Write the exploration music as four stems in the same key and tempo. Keep the pad always on, bring in the bass near danger, and add drums when a fight starts.”
Where you’ve seen it
Red Dead Redemption's score, by Bill Elm and Woody Jackson, was recorded as stems all in A minor at a fixed tempo so they could be overlaid. Rockstar says it was limited to five stem tracks at a time.
Not to be confused with horizontal resequencing
Vertical changes which layers play at the same moment. Horizontal changes which section comes next.